34 GROWTH AND WORK OF PLANTS 



membrane the entire space of the cell is not filled with proto- 

 plasm. There are large spaces filled with a watery fluid called 

 cell sap, which is a solution of certain salts, sugars, etc., in 

 water. If the root hairs are placed in a 5 per cent salt solution, 

 the membrane of protoplasm lies between the cell sap inside 

 of the cell and the salt solution. The cell wall permits the water 

 and solutions to filter through easily. Butjthe protoplasmic 

 membrane^is ofa closer and different texture, so__thal_soluble 

 substances do_jiojjas&. thrmigji so readily. The water of the 

 solution, however, passes through the membrane easily. The 

 result is that the protoplasm contracts away from the cell 

 wall. This is because some of the water in the cell sap flowed 

 through the protoplasmic membrane into the salt solution outside. 

 According to well-known laws of physics, the greater flow of water 

 through such a membrane, which is only half-way permeable 

 (semi-permeable), is always in^he direction of the stronger solu- 

 tion. This shows then that the 5 per cent salt solution is stronger 

 than the cell-sap solution. But when the salt solution is replaced 

 with water, the membrane of protoplasm moves out again against 

 the cell wall and is pressed firmly against it, so that the elastic cell 

 wall becomes slightly stretched while the cell becomes firm and 

 turgid, or is in a state of turgescence, or tension, something like an 

 inflated bladder. This action of the protoplasmic membrane, 

 the cell wall, and the cell sap explains to us how it is that the deli- 

 cate root hairs can take up water and food solutions from the soil. 

 57. The behavior of a root hair or other cell in the absorp- 

 tion of water is sometimes illustrated in the following way.* 

 Over the bulb of a thistle tube a piece of a bladder membrane is 

 tied after thoroughly soaking it. A saturated solution of sugar 

 in water with a small quantity of a red analine dye to color it is 

 poured into the tube to fill the bulb and a short distance into the 

 tube. The bulb of the thistle tube is lowered into a bottle of 

 water so that the height of the water in the bottle and the solution 

 in the tube are at the same level. In a few hours or in a day or so, 

 if the experiment is properly set up, the solution in the tube will 

 * Or by the well-known egg experiment. 



